Planar phased array antennas are known in the art, and are often used in radar and RF communications applications. Such antennas typically comprise a plurality of radiating structures arranged in a planar array, operative to emit electromagnetic energy at RF frequencies, phased so as to form an electronically steerable beam.
Phased array antennas require phase shifting networks or circuits to effect the phase shifting for each radiating structure. Typical phased array antennas include a waveguide or microstrip transmission line signal feed network affixed to one side of a mounting structure associated with a phase shifting subassembly. The feed network directs incoming RF energy into the phase shifting subassembly, where individual phase-shifting elements are provided for each of the plurality of radiating structures. The phase shifting subassembly typically includes a plurality of phase shifting devices which impose a predetermined amount of phase shift upon a signal in accordance with a control signal originating in a control source typically external to the antenna. The radiating structures, such as horns, notch elements, or waveguide elements, are physically affixed to the opposite side of the mounting structure, and are operative to receive the phase-shifted energy from the phase shifting subassembly and emit same outwardly of the plane of the phased array antenna.
Mounting structures for conventional phase shifter subassemblies have been constructed of aluminum for light weight, low cost, and ease of fabrication. The phase shifters themselves are typically ferrite waveguide devices or MIC/MMIC (microwave integrated circuit or monolithic microwave integrated circuit) devices on ceramic substrates.
Difficulties have been encountered in the mounting of phase shifters on conventional mounting structures, in that the ferrite or ceramic with which the phase shifters are constructed has a different coefficient of thermal expansion than the aluminum mounting structure to which the phase shifters are mounted. The mismatch in thermal expansion coefficients between the phase shifters and the mounting structure causes performance degradation and eventual physical failure as a result of thermal cycling. Difficulties due to thermal cycling are especially pronounced in phased array antennas deployed in avionics bays of aircraft, which can experience substantial temperature changes in a short period of time.
Several classes of structural materials are available which have coefficients of thermal expansion closely matched to ferrites and ceramics. Some of the more common materials include titanium, KOVAR.RTM. metal alloy (manufactured by Carpenter Technology Group, Reading, Pa.), INCONELO nickel alloy (manufactured by International Nickel Co. Inc., New York, N.Y.), metal matrix aluminums, and specially doped plastics. However, none of these materials possess a satisfactory combination of the desirable properties of aluminum such as light weight, low material cost, ease of machinability, corrosion resistance, high thermal conductivity, and high electrical conductivity.
Accordingly, there is a need for an improved non-flat mounting structure for phase shifters, amplifiers, and electronic circuits that is reliable, low cost, and easy to fabricate.